In the co-pending, commonly assigned, applications Ser. No. 657,888, filed Oct. 5, 1984, entitled "Cartridge Containing Plasma Source for Accelerating a Projectile" and Ser. No. 809,071, filed Dec. 11, 1985, entitled "Plasma Propulsion Apparatus and Method" there are disclosed an apparatus for and method of accelerating a projectile with a high pressure plasma produced in response to a high voltage discharge. The projectile is located in a gun barrel downstream of a high pressure source of ionized gas including a capillary passage, i.e., a passage having a length to diameter ratio of at least 10:1. The passage includes ionizable material, preferably low atomic weight elements in molecules forming the passage wall. The low atomic weight elements (e.g. hydrogen and carbon) are ablated from the wall in response to a high voltage discharge established between spaced first and second electrodes, respectively located at open and closed ends of the passage. The passage dimensions and ionized materials cause the discharge to have a relatively high resistance, such as 0.1 ohm. A gas having a high pressure, such as in excess of 100 bars, is established in the capillary passage and escapes through the open end of the passage to accelerate a projectile in a gun barrel, usually made of steel, located immediately downstream of the open end.
To maximize the projectile velocity, an electric ionizing pulse supplied to the electrodes is shaped so the pressure behind the projectile in the barrel is maintained substantially constant despite the increasing volume in the barrel behind the moving projectile. The electric pulse is shaped so the power applied to the discharge increases substantially linearly as a function of time while the projectile is being accelerated through the barrel. The pulse is terminated prior to the projectile reaching the muzzle end of the barrel, approximately at the time that the projectile has traversed approximately one-half of the barrel length.
A confined mass of evaporable, ionizable material is located between the open end of the capillary tube and the back end of the projectile. This mass of material is evaporated and then ionized by plasma in the discharge to add additional propulsive force to the projectile and to cool the plasma discharge escaping from the open end of the capillary passage. Preferably, the evaporable material between the open end of the capillary and the back end of the projectile includes low atomic weight elements, such as hydrogen and carbon.
While the prior art structures have functioned satisfactorily for many purposes, they have generally been limited to accelerating projectiles to about 5 kilometers per second because high atomic weight elements, e.g., iron, of the gun barrel are melted and evaporated by the plasma to reduce the sound speed of the projectile accelerating gas. To avoid barrel wall melting, it is necessary to maintain the temperature of gas flowing into the gun barrel to below about 3500.degree. K. This limitation of 3500.degree. K. translates into a projectile velocity limitation of about 5 kilometers per second for hydrogen-rich flows.
It is, accordingly, an object of the present invention to provide a new and improved apparatus for and method of accelerating a projectile through the use of electric discharge plasmas.
Another object of the invention is to provide a new and improved apparatus for and method of enabling high pressure gases to be built up behind a projectile in response to a plasma discharge that initiates the plasma.
Another object of the invention is to provide a new and improved apparatus for and method of accelerating projectiles to speeds in excess of 10 kilometers per second.
An additional object of the present invention is to provide a new and improved apparatus for and method of accelerating projectiles wherein the projectile velocity is not limited by melt characteristics of an elongated barrel downstream of a high pressure, high temperature plasma source.